1. Field of the Invention
The invention relates to a method for operating a power plant with a fossil-fueled steam generator, in which heat contained in flue gas from a furnace is utilized to generate steam for a steam turbine and nitrogen is removed from the hot flue gas, preheated feedwater at high pressure is evaporated, and steam being produced is superheated prior to entry into the steam turbine and after partial depressurization or expansion in the steam turbine. The invention is also directed to a power plant operated by this method.
In such a power plant, which is also referred to as a steam power plant, heating surfaces of the fossil-fueled steam generator are connected into a water-steam loop of the steam turbine. The tubes joined together in gas-tight fashion to form a combustion chamber wall of the steam generator form an evaporator heating surface, which is connected with the other heating surfaces that are also disposed inside the steam generator. The further heating surfaces are typically a high-pressure superheater or economizer for preheating the feedwater and a high-pressure superheater for final superheating of the steam being generated, as well as an intermediate superheater for re-superheating the partially depressurized or expanded steam in a high-pressure portion of the steam turbine.
The steam generation is effected by transferring the heat contained in the flue gas from the furnace to the medium flowing in the water-steam loop. In order to achieve the highest possible efficiency of the power plant, the heating surfaces are disposed in different temperature regions of the steam generator, for the sake of adaptation to the temperature course of the flue gas. Typically, in terms of the flow direction of the flue gas, the intermediate superheater is disposed downstream of the high-pressure superheater, and upstream of the economizer.
A power plant having such a heating surface configuration inside the steam generator is known from European Patent No.0 054 601 B1, for example. In that known power plant, in addition to the economizer, two further high-pressure preheaters are provided upstream, inside the water-steam loop and outside the steam generator. The fresh steam state achieved thus far, that is the temperature and the pressure of the steam upon its entry into the steam turbine, is at a pressure of 250 bar at maximum and a temperature of 545.degree. C. at maximum.
In a power plant having a nitrogen removal system or device (deNO.sub.x device) operating by the principle of selective catalytic reduction (SCR process), the device is typically disposed inside the steam generator downstream of the economizer in the flow direction of the flue gas. Since the temperature of the flue gas inside the steam generator and therefore in the region of the nitrogen removal system varies as well when load changes take place in the power plant, the temperature drops below the operating temperature of the nitrogen removal system, of approximately 300.degree. to 350.degree. C., in various operating states, particularly in the partial load range. In that case, adequate flue gas cleaning is no longer possible.
In order to assure adequate cleaning of the flue gas even if the flue gas temperature downstream of the economizer drops below the operating temperature of the deNO.sub.x device, a so-called ECO bypass is provided in accordance with a circuit known from a publication entitled: "Chemie-Technik" [Chemical Technology], Vol. 15, No. 2, 1986, pp. 17 ff., particularly FIG. 3 on page 18. Through that bypass, an adjustable partial flow of flue gas withdrawn upstream of the economizer is admixed with the flue gas downstream of the economizer. Thus the flue gas temperature, for instance in partial load operation, is increased accordingly in the region of the nitrogen removal system. However, with that provision, which involves especially major technological effort and expense, the reaction temperature for the nitrogen removal system can merely be kept in the vicinity of an especially advantageous value.